In a high-speed particle generating apparatus for generating high-speed particles by irradiating a pulsed laser beam to a high-speed particle generating target, it is necessary to condense a laser beam to a micro-spot on the target surface in order to generate the high-speed particles efficiently. However, there are many cases that in a high-intensity laser beam, the laser beam itself to be outputted has been already distorted. In addition to the beam diameter expanding due to distortion of the wave front generated during the propagation, the wave front also may be distorted due to thermal deformation of the optical system. As a result, there are some cases that the spot diameter of the laser beam at a condensing point (condensing position) is not reduced.
Conventionally, in order to enhance an optical intensity by converging a laser beam to a micro-spot, a wave front compensation control adjusting a wave front of the laser beam is conducted. For example, in Katsuaki Akaoka et al., “Closed loop wavefront correction of Ti: sapphire Chirped Pulse Amplification laser beam,” SPIE, Vol. 3265, 29–30, Jan. 1998, p. 219–225, a technique for compensating for the wave front of a laser beam is disclosed to set a He—Ne laser as a reference light source in an optical path of a high-intensity laser, measure the wave front outputted from the reference light source at a wave front sensor, and stores its measurement results as a reference wave front, so that the wave front of the laser beam from a titanium sapphire laser is conformed with the reference wave front.